COS 236-1 - Variation in sediment blue carbon stocks in coastal wetlands from the Pacific coast of North America

Abstract: Coastal wetlands including seagrass meadows, tideflats, emergent marshes, mangroves, and temperate forested swamps support a variety of key ecological functions and services throughout the world. Per unit area, these ecosystems are highly effective at sequestering and storing organic carbon (termed ‘blue carbon’), principally in their sediments. Blue carbon services provide an additional motivation to conserve and restore these valuable ecosystems. The Pacific Northwest Blue Carbon Working Group conducted a regional synthesis to assemble available data on blue carbon stocks from intertidal and shallow subtidal wetlands distributed from Alaska to Mexico. We collected approximately 70 published and unpublished datasets, comprising about 1250 discreet soil profiles, to evaluate differences in stocks among wetland classes, relationships between soil stocks and environmental drivers, and spatial variation in blue carbon. We found that sediment carbon stocks were lowest in seagrass meadows and tideflats (which had similar carbon reservoirs), several fold higher in emergent marshes, and highest in tidal wetlands dominated by woody species (mangroves in Mexico and temperate tidal swamps from central California to the Pacific Northwest). Stocks from seagrass and marshes tended to vary little with latitude across the west coast, but there were trends towards decreasing stocks at higher latitudes in other wetland classes. For both marshes and tidal swamps, stocks increased with elevation in the intertidal zone. Our data suggest that for the most well studied blue carbon wetland type – emergent marshes – local scale drivers such as elevation may be more important than broader scale biogeographic factors at influencing sediment stocks. Synthesis of regional data sets on coastal blue carbon will assist with efforts to conduct carbon accounting, and will help wetland managers evaluate expected blue carbon services for different wetland restoration and conservation scenarios.